Sample rate conversion module and applications thereof

ABSTRACT

A sample rate converter includes an upsampling module, a low pass filter, and a linear sample rate conversion module. The upsampling module is operably coupled up-sample a digital input signal having a first rate to produce a digitally up-sampled signal. The low pass filter is operably coupled to low pass filter the digitally up-sampled signal to produce a digitally filtered signal at an up-sampled rate. The linear sample rate conversion module is operably coupled to convert the digitally up-sampled signal into a sample rate adjusted digital signal having a second rate based on an control feedback signal and a linear function, wherein a relationship between the first rate and the second rate is a non-power of two.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

This invention relates generally to portable handheld digital audiosystems and more particularly to integrated circuits comprising ahandheld audio system.

2. Description of Related Art

As is known, handheld digital audio systems are becoming very popular.Such systems include digital audio players/recorders that record andsubsequently playback MP3 files, WMA files, etc. Such digital audioplayers/recorders may also be used as digital dictaphones and filetransfer devices. Further expansion of digital audio players/recordersincludes providing a frequency modulation (FM) radio receiver such thatthe device offers FM radio reception.

While digital audio players/recorders are increasing their feature sets,the increase in feature sets has been done in a less than optimalmanner. For instance, with the inclusion of an FM receiver in a digitalaudio player/recorder, the FM receiver is a separate integrated circuitfrom the digital audio player/recorder chip set, or IC. As such, the FMreceiver integrated circuit (IC) functions completely independently ofthe digital audio player/recorder IC, even though both ICs include somecommon functionality.

Further, FM receivers function to convert a radio frequency signal intoa complex baseband signal that is decoded, or demodulated, to produce adecoded baseband signal. A channel separation function splits thedecoded baseband signal into a left channel signal and a right channelsignal. As with any integrated circuit implemented using CMOS(complimentary metal oxide semiconductor) technology, errors in timing,signal recapture, crystal frequency variations, and variations from partto part occur. To account for such errors, FM receivers include errorcorrecting circuit to adjust the decoded baseband signal once it isproduced. While this technical reduces the above described adverseaffects, it requires a fairly complex circuit implementation andaddresses the problem in a passive cause and affect manner.

Four papers teach FM receivers that address at least one of the abovementioned issues. The four papers include, “A 10.7-MHz IF-to-BasebandSigma-Delta A/D Conversion System for AM/FM Radio Receivers” by Eric VanDer Zwan, et. al. IEEE Journal of Solid State Circuits, VOL. 35, No. 12,December 2000; “A fully Integrated High-Performance FM Stereo Decoder”by Gregory J. Manlove et. al, IEEE Journal of Solid State Circuits, VOL.27, No. 3, March 1992; “A 5-MHz IF Digital FM Demodulator”, by JaejinPark et. al, IEEE Journal of Solid State Circuits, VOL. 34, No. 1,January 1999; and “A Discrete-Time Bluetooth Receiver in a 0.13 μmDigital CMOS Process”, by K. Muhammad et. al, ISSCC2004/Session15/Wireless Consumer ICs/15.1, 2004 IEEE International Solid-StateCircuit Conference.

Therefore, a need exists for a method and apparatus for radio decodingthat is optimized to function with a digital audio player/recorder andincludes sample rate conversion.

BRIEF SUMMARY OF THE INVENTION

The sample rate conversion module and applications thereof of thepresent invention substantially meet these needs and others. In oneembodiment, a sample rate converter includes an upsampling module, a lowpass filter, and a linear sample rate conversion module. The upsamplingmodule is operably coupled to up-sample a digital input signal having afirst rate to produce a digitally up-sampled signal. The low pass filteris operably coupled to low pass filter the digitally up-sampled signalto produce a digitally filtered signal at an up-sampled rate. The linearsample rate conversion module is operably coupled to convert thedigitally up-sampled signal into a sample rate adjusted digital signalhaving a second rate based on a control feedback signal and a linearfunction, wherein a relationship between the first rate and the secondrate is a non-power of two.

In another embodiment, a sample rate converter includes a samplingmodule, a determining module, and an outputting module. The samplingmodule is operably coupled to sample, at a sampling rate, an inputsignal to produce a digital sampled signal at the sampling rate. Thedetermining module is operably coupled to determine whether, for a givensample of an interpolated digital signal at a second rate, a sample ofthe digital sampled signal has an error term within a first value range,wherein the second rate is less than the sampling rate and arelationship between the sampling rate and the second rate is anon-power of two. The outputting module is operably coupled to pass thesample of the digital sampled signal as the given sample when the errorterm is within the first value range and to determine a value for thegiven sample based on the sample of the digital sampled signal, aprevious sample of the digital sampled signal, and the error term whenthe error term is not within the first value range.

In yet another embodiment, a digital Frequency Modulation (FM) decoderincludes a low noise amplifier, a mixing module, an analog to digitalconversion module, a digital baseband conversion module, a sample rateconverter, a demodulation module, a channel separation module, and anerror sensing module. The low noise amplifier is operably coupled toamplify a received FM signal to produce an amplified FM signal. Themixing module is operably coupled to convert the amplified FM signalinto a low intermediate frequency (IF) signal based on a localoscillation. The analog to digital conversion module is operably coupledto convert the low IF signal into a digital low IF signal. The digitalbaseband conversion module is operably coupled to convert the digitallow IF signal into a digital baseband signal. The sample rate converteris operably coupled to adjust the digital baseband signal from a firstrate to a second rate based on a feedback error signal to produce adigital FM encoded signal. The demodulation module is operably coupledto demodulate the digital FM encoded signal to produce a digital FMcomposite signal. The channel separation module is operably coupled toseparate a left channel signal and a right channel signal from thedigital FM composite signal. The error sensing module is operablycoupled to produce the feedback error signal based on the digital FMcomposite signal.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a handheld audio system inaccordance with the present invention;

FIG. 2 is a schematic block diagram of another embodiment of a handheldaudio system in accordance with the present invention;

FIG. 3 is a schematic block diagram of yet another embodiment of ahandheld audio system in accordance with the present invention;

FIG. 4 is a logic diagram of a method performed by a digital radiointerface in accordance with the present invention;

FIG. 5 is a timing diagram illustrating the interconnectivity of a radiosignal decoder and digital audio processing integrated circuit inaccordance with the present invention;

FIG. 6 is a schematic block diagram of a radio signal decoder integratedcircuit in accordance with the present invention;

FIG. 7 is a schematic block diagram of another embodiment of a radiosignal decoder integrated circuit in accordance with the presentinvention;

FIG. 8 is a schematic block diagram of a radio signal decoder inaccordance with the present invention;

FIG. 9 is a frequency spectrum diagram of a digital radio compositesignal in accordance with the present invention;

FIG. 10 is a logic diagram illustrating the functionality of an errorsensing module in accordance with the present invention;

FIG. 11 is a schematic block diagram of an error sensing module inaccordance with the present invention;

FIG. 12 is a schematic block diagram of a feedback module in accordancewith the present invention;

FIG. 13 is a schematic block diagram of a decoder in accordance with thepresent invention;

FIG. 14 is a schematic block diagram of another embodiment of a decoderin accordance with the present invention;

FIG. 15 is a schematic block diagram of a digital decoder in accordancewith the present invention;

FIG. 16 is a diagram of an example of error correction in accordancewith the present invention;

FIG. 17 is a schematic block diagram of a sample rate conversion modulein accordance with the present invention;

FIG. 18 is a schematic block diagram of another embodiment of a samplerate converter in accordance with the present invention; and

FIGS. 19A-19D illustrate an example of sample rate conversion,demodulation and error sensing in accordance with the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a handheld audio system 10 thatincludes a radio signal decoder integrated circuit 12 and a digitalaudio processing integrated circuit 14. The digital audio processingintegrated circuit 14 includes a processing module 13, memory 15, and aDC-to-DC converter 17. The processing module 13 may be a singleprocessing device or a plurality of processing devices. Such aprocessing device may be a microprocessor, micro-controller, digitalsignal processor, microcomputer, central processing unit, fieldprogrammable gate array, programmable logic device, state machine, logiccircuitry, analog circuitry, digital circuitry, and/or any device thatmanipulates signals (analog and/or digital) based on operationalinstructions. The memory 15 may be a single memory device or a pluralityof memory devices. Such a memory device may be a read-only memory,random access memory, volatile memory, non-volatile memory, staticmemory, dynamic memory, flash memory, cache memory, and/or any devicethat stores digital information. Note that when the processing module 13implements one or more of its functions via a state machine, analogcircuitry, digital circuitry, and/or logic circuitry, the memory storingthe corresponding operational instructions may be embedded within, orexternal to, the circuitry comprising the state machine, analogcircuitry, digital circuitry, and/or logic circuitry. Further note that,the memory 15 stores, and the processing module 13 executes, operationalinstructions corresponding to at least some of the steps and/orfunctions illustrated in FIGS. 1-19.

In an embodiment, when a battery (e.g., V_battery 19), or other externalpower source, is initially applied to the radio signal decoder 12, whichwill be described in greater detail with reference to FIGS. 3-19, andthe digital audio processing IC 14, the DC-DC converter 17 generates apower supply voltage 24 based on an internal oscillation. When the powersupply voltage 24 reaches a desired value (e.g., near a regulatedvalue), the processing module 13 provides an enable signal 20 (which islabeled as optional 20) to the radio signal decoder IC 12. In responseto the enable signal 20, the radio signal decoder IC 12 generates thesystem clock 22; with the remaining functionality of the radio signaldecoder 12 being inactive awaiting a second enable signal or beingactivated once the system clock 22 is functioning. The radio signaldecoder 12 provides the system clock 22 to the audio processingintegrated circuit 14. Upon receiving the system clock 22, the DC-DCconverter may switch from the internal oscillation to the system clock22 to produce the power supply voltage 24 from the battery voltage 19,or external power source. Note that when the radio signal decoder 12 ispowered via the battery (V_battery 19), it may produce a real time clock(RTC) in addition to producing the system clock 22.

In another embodiment, when the battery is initially applied to thedigital audio processing IC 14 and the DC-DC converter is enabled, theDC-DC converter generates a power supply voltage 24. The DC-DC converter17 provides the power supply voltage 24 to circuit modules within thedigital audio processing IC 14 and to the radio signal decoder IC 12. Apower enable module 95 monitors to the power supply voltage 24 and whenit reaches a desired value (e.g., at or near a steady state value), thepower enable module 95 generates the enable signal 20. The radio signaldecoder IC 12 generally responds to the enable signal 20 as discussed inthe previous paragraph.

With the system clock 22 functioning, the radio signal decoder IC 12converts a received radio signal 16 into left and right channel signals18, which may be analog or digital signals. In one embodiment, the leftand right channel signals 18 include a Left+Right signal and aLeft−Right signal. The radio signal decoding IC 12 provides the left andright channel signals 18 to the digital audio processing IC 14.

The digital audio processing integrated circuit 14, which may be adigital audio player/recorder integrated circuit such as the STMP35XXand/or the STMP36XX digital audio processing system integrated circuitsmanufactured and distributed by Sigmatel Incorporated, receives the leftand right channel signals 18 and produces therefrom audio signals 26.The digital audio processing IC 14 may provide the audio signals 26 to aheadphone set or other type of speaker output. As an alternative toproducing the audio signals 26 from the left and right channel signals18, the digital audio processing integrated circuit 14 process storedMP3 files, stored WMA files, and/or other stored digital audio files toproduce the audio signals 26.

FIG. 2 is a schematic block diagram of another handheld audio system 40that includes the radio signal decoder integrated circuit 12 and thedigital audio processing integrated circuit 14. In this embodiment, theradio signal decoder integrated circuit 12 includes an antenna structure34 and a crystal oscillator circuit 30, which is operably coupled to acrystal 25 (e.g., a 24 MHz crystal). The crystal oscillation circuit 30is operably coupled to produce a reference oscillation 32 from thecrystal 25. The radio signal decoder integrated circuit 12, which mayinclude one or more phase locked loops, converts the referenceoscillation 32 into an oscillation from which the system clock 22 isderived. For example, the system clock 22 may be the output oscillationof a phase locked loop, an oscillation that is a multiple or fraction ofthe output oscillation of the phase locked loop.

The antenna structure 34 includes an antenna, a plurality of capacitors,and an inductor coupled as shown. The receive radio signal 16 isprovided from the antenna structure 34 to the radio signal decoderintegrated circuit 12. As with the embodiment of FIG. 1, the radiosignal decoder integrated circuit 12 converts the receive radio signal16 into left and right channel signals 18.

The digital audio processing integrated circuit 14, via the DC-DCconverter 17, generates an input/output (I/O) dependent supply voltage24-1 and an integrated circuit (IC) dependent voltage 24-2 that aresupplied to the radio signal decoder IC 12. In one embodiment, the I/Odependent voltage 24-1 is dependent on the supply voltage required forinput/output interfacing of the radio signal decoder IC and/or thedigital audio processing IC 14 (e.g., 3.3 volts) and the IC dependentvoltage 24-2 is dependent on the IC process technology used to produceintegrated circuits 12 and 14. In an embodiment, the integrated circuitprocess technology is 0.08 to 0.35 micron CMOS technology where the ICdependent voltage 24-2 is 1.8 volts or less.

The interface between the integrated circuits 12 and 14 further includesa bi-directional interface 36. Such an interface may be a serialinterface for the integrated circuits 12 and 14 to exchange control dataand/or other type of data, including the enable signal 20. In oneembodiment, the bi-directional interface 36 may be one or more serialcommunication paths that are in accordance with the I²C serialtransmission protocol. As one or ordinary skill in the art willappreciate, other serial transmission protocols may be used for thebi-directional interface 36 and the bi-directional interface 36 mayinclude one or more serial transmission paths.

FIG. 3 is a schematic block diagram of yet another embodiment of thehandheld audio system 50 that includes the radio signal decoderintegrated circuit 12 and the digital audio processing integratedcircuit 14. In this embodiment, each of the radio signal decoderintegrated circuit 12 and the digital audio processing IC 14 includes adigital radio interface 52. The digital radio interface 52 is operablycoupled to provide the left and right channel signals 18 from the radiosignal decoding IC 12 to the digital audio processing integrated circuit14. Within the radio signal decoder IC 12, the digital radio interface52 converts parallel left and right channel signals 18 into a serialsignal and, within the digital audio processing integrated circuit 14,the digital radio interface 52 converts the serial left and rightchannel signals 18 back into parallel signals. Note that the serial toparallel and parallel to serial functionality of the digital radiointerface 52 may be programmable based on the sample rate of the radiosignal decoder integrated circuit 12, a desired data rate, or otherparameters of the ICs 12 and 14.

In general, the digital radio interface 52 is a custom interface forconnecting the digital audio processing integrated circuit 14 to theradio signal decoder IC 12. Such a digital radio interface 52 maygenerate a data clock of 4 MHz or 6 MHz, or some other rate, to supportthe conveyance of serial data between the ICs 12 and 14. In addition,the digital radio interface 52 formats the serial data into a packet, orframe, that includes one to five data words having a sampling rate basedon the sample rate conversion of the radio signal decoder IC 12, whichwill be described in greater detail with reference to FIGS. 8-19.Nominally, a packet, or frame, will include four 18-bit words having asampling rate of at 44.1 KHz per word, 2 of the 18 bits are for controlinformation and the remaining 16 bits are for data.

The digital radio interface 52 may convey more that the left and rightchannel signals 18, which may be in the form of Left+Right channelsignals and Left−Right channel signals. For instance, the digital radiointerface 52 may convey receive signal strength indications, data clockrates, control information, functionality enable/disable signals,functionality regulation and/or control signals, and radio data servicesignals between the ICs 12 and 14.

FIG. 4 is a logic diagram of the functionality of the digital radiointerface 52. In this embodiment, the digital radio interface 52determines the first and second actual sampling rates of a signal to beconveyed to the digital audio processing integrated circuit (Step 60).At Step 62, the digital radio interface utilizes the first and secondactual sampling rates to achieve, over time, a given sampling rate thatcorresponds to the desired output sampling rate.

FIG. 5 illustrates a timing diagram of data transmission via the serialinterconnection between the digital radio interfaces of integratedcircuits 12 and 14. As shown, a sample rate conversion ready signal(SRC_RDY) 70 is periodically activated. Clock signal 72 corresponds tothe data clock that is derived from the system clock 22. The rate forclock 72 may range from a few megahertz to tens of megahertz and beyond.

From the SRC_RDY signal 70 and clock 72 the digital radio interfacegenerates a DRI_clock 74. The DRI_clock 74 includes a clocking portion,which has a frequency corresponding to clock 72, and a plurality ofquiet periods (Q). The last quiet period between sample rate readysignals pulses is designated as the final quiet period (QF). The quietperiods correspond to a rate of the data ready, or sample rateconversion ready signal 70, and the rate of clock signal 72.

Serial data 76 is transmitted between the integrated circuits 12 and 14in accordance with the DRI_clock 74. During the quiet periods (Q), nodata is transmitted. As such, serial data 76 is only transmitted whenthe DRI_clock 74 is active. The serial data 76 includes one or morewords (e.g., 1-5 words), where each word includes 18 bits, 2 of whichare used for control information 80 and the remaining 16 bits are fordata 78. The formatting of the serial data may be in accordance with oneor more serial data transmission protocols (e.g., I²C).

FIG. 6 is a schematic block diagram of an embodiment of the radio signaldecoder integrated circuit 12 that includes the digital radio interface52, a crystal oscillation circuit (XTL OSC CKT) 94, a phase locked loop(PLL) 92, the power enable module 95, and a radio signal decoder 90. Thecrystal oscillation circuit 94 is operably coupled, via integratedcircuit pins, to an external crystal 96 to produce a referenceoscillation 108. The rate of the reference oscillation 108 is based onthe properties of the external crystal 96 and, as such, may range from afew mega-Hertz to hundreds of kilo-Hertz. In an embodiment, thereference oscillation 108 produces the system output clock 110, which isoutputted via a clock output (CLK_out) pin 102. As one of ordinary skillin the art will appreciate, the system clock 110 may be identical to thereference oscillation 108, may have a rate that is a multiple ofreference oscillation 108 via the rate adjust module 93, may have a ratethat is a fraction of reference oscillation 108 via the rate adjustmodule 93, may have a phase shift with respect to the referenceoscillation, or a combination thereof.

The phase locked loop 92 also produces a local oscillation 106 from thereference oscillation 108. The rate of the local oscillation correspondsto a difference between an intermediate frequency (IF) and a carrierfrequency of the received radio signal 16. For instance, if the desiredIF is 2 MHz and the carrier frequency of the received radio signal 16 is101.5 MHz, the local oscillation is 99.5 MHz (i.e., 101.5 MHz−2 MHz). Asone of ordinary skill in the art will appreciate, the intermediatefrequency (IF) may range from DC to a few tens of MHz and the carrierfrequency of the received radio signal 16 is dependent upon theparticular type of radio signal (e.g., AM, FM, satellite, cable, etc.).As one of ordinary skill in the art will further appreciate, the radiosignal decoder 90 may process a high side carrier or a low side carrierof the RF signals and/or IF signals.

The radio signal decoder 90 converts the received radio signal 16, whichmay be an AM radio signal, FM radio signal, satellite radio signal,cable radio signal, into the left and right channel signals 18 inaccordance with the local oscillation 106. The radio signal decoder 90,which will be described in greater detail with reference to FIGS. 8-19,provides the left and right channel signals to the digital radiointerface 52 for outputting via a serial output pin 104. The serialoutput pin 104 may includes one or more serial input/output connections.As is further shown, the radio signal decoder 90 may receive the enablesignal 20 via a power-up pin 98 and a power supply voltage from powersupply pin 100. Alternatively, the power enable module 95 generates theenable signal 20 when the power supply 24 reaches a desired value. Inthis instance, IC pin 98 may be used for another function.

FIG. 7 is a schematic block diagram of another embodiment of the radiosignal decoder integrated circuit 12. In this embodiment, the integratedcircuit 12 includes the digital radio interface 52, the crystaloscillation circuit 94, the phase locked loop 92, the optional rateadjust module 93, and the radio signal decoder 90. As is further shown,the integrated circuit 12 includes a plurality of integrated circuitpins: the serial output pin 104, the clock out pin 102, an IC dependentsupply pin 100-1, an I/O dependent supply voltage pin 100-2, abi-directional pin 122, and a serial data clock pin 120. The serial dataclock pin 120 supports a serial data clock that is transmitted betweenintegrated circuit 12 and integrated circuit 14 and the bi-directionalpin 122 supports transmission of bi-directional data between integratedcircuit 12 and integrated circuit 14.

FIG. 8 is a schematic block diagram of a radio signal decoder 90 thatincludes a low noise amplifier (LNA) 130, a mixing module 132, ananalog-to-digital conversion module 134, a digital baseband conversionmodule 136, a sample rate conversion module 138, a demodulation module140, a channel separation module 142, and an error sensing module 144.

In operation, the low noise amplifier 130 receives the radio signal 16and amplifies it to produce an amplified radio signal 146. The gain atwhich the low noise amplifier 130 amplifies the receive signal 16 isdependent on the magnitude of the received radio signal 16 and automaticgain control (AGC) functionality of the radio signal decoder 90. Themixing module 132 mixes the amplified radio signal 146 with the localoscillation 106 to produce a low intermediate frequency signal 148. Ifthe local oscillation 106 has a frequency that matches the frequency ofthe radio signal 146 the low intermediate frequency signal 148 will havea carrier frequency of approximately zero. If the local oscillation 106is slightly more or less than the radio signal 146, then the lowintermediate frequency signal 148 will have a carrier frequency based onthe difference between the frequency of the radio signal 146 and thefrequency of local oscillation 106. In such a situation, the carrierfrequency of the low IF signal 148 may range from 0 hertz to tens ofmega-Hertz.

The analog-to-digital conversion module 134 converts the low IF signal148 into a digital low IF signal 150. In one embodiment, the low IFsignal 148 is a complex signal including an in-phase component and aquadrature component. Accordingly, the analog-to-digital conversionmodule 134 converts the in-phase and quadrature components of the low IFsignal 148 into corresponding in-phase and quadrature digital signals150.

The digital baseband conversion module 136 is operably coupled toconvert the digital low IF signals 150 into digital baseband signals152. Note that if the digital low IF signals 150 have a carrierfrequency of zero, the digital baseband conversion module 136 primarilyfunctions as a digital filter to produce a digital baseband signals 152.If, however, the intermediate frequency is greater than zero, thedigital baseband conversion module 136 functions to convert the digitallow IF signals 150 to have a carrier frequency of zero and performsdigital filtering.

The sample rate conversion module 138, which will be described ingreater detail with reference to FIGS. 17-19, receives the digitalbaseband signal 152 and a feedback error signal 154 to produce a digitalradio encoded signal 156. The demodulation module 140 demodulates thedigital radio encoded signal 156 to produce a digital radio compositesignal 158. The error sensing module 144, which will be described ingreater detail with reference to FIGS. 10-12, interprets the radiosignal composite signal 158 to produce the feedback error signal 154.The channel separation module 142 is operably coupled to produce theleft and right channel signals 18 from the digital radio compositesignal 158.

FIG. 9 is a frequency diagram of the digital radio composite signal 158.The signal includes a pilot tone at 19 KHz and another tone at 38 KHz.The signal 158 also includes a low frequency left plus right (L+R)signal component, a left minus right (L-R) signal component, and a radiodata signal (RDS) signal component. The error sensing module 144utilizes the known properties of the 19 KHz pilot tone and thecorresponding properties of the actual pilot tone embedded within thedigital composite radio signal 158 to determine the error feedbacksignal 154. In such an embodiment, the sample rate conversion module 138removes errors due to process variation, temperature variations, etcetera from the digital baseband signals 152 prior to demodulation viathe demodulation module 140. As such, the demodulation errors of priorart embodiments are avoided by correcting this signal prior todemodulation.

FIG. 10 is a logic diagram illustrating the functionality of the errorsensing module. The processing of the error sensing module begins atStep 160 where it determines a period of the decoded radio compositesignal based on a known property of the signal. For example, knownproperty may be a pilot tone (e.g., 19 KHz or 38 KHz), a trainingsequence (e.g., a preamble of known tones), an auto correlationfunction, and/or a cross correlation function.

The processing then proceeds to Step 162 where the error sensing modulecompares the measured period of the decoded radio composite signal withan ideal period of the radio composite signal. For example, the errorsensing module compares the measured frequency of the 19 KHz pilot tonewith the known ideal period of the 19 KHz pilot tone.

The processing then proceeds to Step 164 where the error sensing modulegenerates an error feedback signal based on a difference between themeasured period and the ideal period. For example, if the actual periodof the pilot tone is not within acceptable margins (e.g., ±1% or less)of the 19.1 KHz ideal pilot tone, the error sensing module generates anerror feedback signal to indicate the phase and/or frequency differencebetween the measured period of the pilot tone and the ideal period ofthe pilot tone.

FIG. 11 is a schematic block diagram of an embodiment of the errorsensing module 144. In this embodiment, the error sensing module 144includes a mixing module 170, a low pass filter 172, a comparator 174and a feedback module 176. The mixing module 170 mixes a digitalreference oscillation 178 (e.g., a 19.1 KHz tone to represent the idealpilot tone) with the digital radio composite signal 158. The mixingmodule 170, which may include a digital mixer, produces a mixed signal180 (e.g., sin(ω₁t)*sin(ω₂t)=½ cos(ω₁−ω₂)t−½ cos(ω₁+ω₂)t, where ω₁represent 2π*f of the ideal pilot tone and ω₂ represents 2π*f of themeasured pilot tone). The low pass filter 172, which may be amulti-order cascaded integrated cone filter having a 2^(n) down samplingfactor, filters the mixed signal 180 to produce a near-DC feedback errorsignal 182 (e.g., filters out the −½ cos(ω₁+ω₂)t term and passes the ½cos(ω₁−ω₂)t term).

The comparator 174 compares the near DC feedback error signal 182 withthe DC reference 184 to produce an offset 186 (e.g., determines thedifference between ω₁ & ω₂ to produce the offset). If the frequency ofthe composite signal 156 matches the frequency of the digital referenceoscillation 178, the near DC feedback error signal 182 will have a zerofrequency such that the offset 186 will be zero. If, however, thefrequency of the composite signal 158 does not substantially match thefrequency of the digital reference oscillation 178, the near DC feedbackerror signal 182 will have a non-DC frequency. The offset 186 reflectsthe offset of the near DC error feedback signal from DC. The feedbackmodule 176, which will be described in greater detail with reference toFIG. 12, converts the offset 186 into the error feedback signal 154.

FIG. 12 illustrates a schematic block diagram of feedback module 176that includes a state variable filter 190, a summing module 192 and aSigma Delta modulator 194. The state variable filter 190 filters theoffset 186 to produce a filtered offset 196. The state variable filter190 is analogous to a loop filter within a phase locked loop thatincludes a resistive term and a capacitive term to integrate the offset186. In essence, the state variable filter 190 stores the offset 186 asthe filtered offset 196. Note, however, that the state variable filter190 does not set the bandwidth of the error sensing module; its primaryfunction is to act as a low pass filter and memory to store the filteredoffset 96.

The summing module 192 sums the filtered offset 196 with a timingdifference signal 198 to produce a summed signal 200. The timingdifference signal 198 is a known timing difference signal such that thefiltered offset signal 196 represents only the unknown timingdifferences in the system due to such things that include processtolerance and temperature drift. The Sigma Delta modulator 194 quantizesthe summed signal 200 to produce the feedback error signal 154.

FIG. 13 is a schematic block diagram of a decoder 210 that may beutilized within the radio signal decoder integrated circuit 12 or may bea stand-alone decoder for decoding digitally encoded signals that aretransmitted from a separate device. In this embodiment, the decoder 210includes the sample rate conversion module 138, a decoding module 212,and the error sensing module 114. The sample rate conversion module 138is operably coupled to convert, based on the error feedback signal 154,the rate of an encoded signal 214 from a first rate to a second rate toproduce a rate adjusted encoded signal 216. For example, the encodedsignal 214 may have a sampling rate of 400 KHz and the rate adjustedencoded signal 216 may have a sampling rate of 152 KHz or 228 KHz.

The decoding module 212 is operably coupled to decode the rate adjustedencoded signal 216 to produce a decoded signal 218. The functionality ofdecoding module 212 corresponds to the encoding function used to producethe encoded signal 214. Accordingly, if the encoded signal is producedby a modulation function (e.g., AM, FM, BPSK, QPSK, et cetera), thedecoding modulation would be the corresponding demodulation function.Alternatively, if the encoded signal 214 was produced by an encodingfunction, such as scrambling, interleaving, et cetera the decodingmodule would have the corresponding inverse function.

The error sensing module 144 determines the error feedback signal 154based on a difference between a known property of decoded signal 218 andthe actual measured property of decoded signal 218. In one embodiment,the known property of decoded signal 218 corresponds to the period of asignal component of the decoded signal 218. This period is compared withthe ideal period of that signal component to produce the error signal154. The signal component may comprise a pilot tone and/or trainingsequence.

FIG. 14 is a schematic block diagram of another embodiment of a decoder220, which may be used within the radio signal decoder integratedcircuit 12 or as a stand-alone decoder. The decoder 220 includes asampling module 222, the sample rate conversion module 138, the decodingmodule 212, and the error sensing module 144. The sample rate conversionmodule 138, decoding module 212 and error sensing module 144 function aspreviously described with reference to FIG. 13.

The sampling module 222 receives an input signal 224 and samples it at agiven sampling rate to produce the encoded signal 214. The input signal224 may be a digital signal or analog signal. If the input signal 224 isan analog signal, the sampling module 222 includes an analog-to-digitalconversion function to produce the encoded signal 214 at the givensampling rate. In general, the decoder functions to receive the inputsignal, which is generated with respect to a first clock domain (e.g.,the clock domain of the transmitter). Sampling module 222 samples theinput signal with a second clock domain and the DRC coverts the samplesfrom the rate of the second clock domain to the rate of the first cockdomain. The decoding module 212 then processes the data at the rate ofthe first clock domain.

FIG. 15 is a schematic block diagram of another embodiment of a digitaldecoder 230 that may be used in the radio signal decoder integratedcircuit 12, or stand-alone decoder. The digital decoder 230 includes afirst sample rate conversion module 138-1, a second sample rateconversion module 138-2, a demodulation module 232, and an error sensingmodule 234. The first sample rate conversion module 138-1 is operablycoupled to adjust the sampling rate of an in-phase (I) signal 236 toproduce a rate adjusted in-phase signal 240, and/or derivative thereof,based on an error feedback signal 244. The second sample rate conversionmodule 138-2 is operably coupled to adjust the sampling rate of aquadrature (Q) signal 238 to produce a sample adjusted quadrature signal242, and/or derivative thereof, based on the error feedback signal 244.As one of average skill in the art would appreciate, the in-phase andquadrature signals 236 and 238 may correspond to signal components ofthe digital baseband signal 152 of FIG. 8.

The demodulation module 232, which may be the demodulation module 140 ofFIG. 8, demodulates the rate adjusted in-phase signal component 240 andrate adjusted quadrature signal component 242 to produce a compositedigital signal. The error sensing module 234, which may correspond tothe error sensing module 144 of FIG. 8, determines the error feedbacksignal 244 based on actual and known properties of the composite digitalsignal. The determination of the error feedback signal 244 may be donein accordance with the previous discussions of the functionality oferror sensing module 144.

FIG. 16 is an example of the functionality of error correction performedby the error feedback module 144, sample rate conversion module 138 anddemodulation module 140. In this illustration, an ideal pilot tone 240is shown as a solid line while actual pilot tone measurements 241 areindicated by dash lines. The error sensing module 144 determines a plusor minus timing error 242 or 244 of the actual pilot tone signal 241with respect to the ideal pilot tone signal 240. The feedback errorsignal 154 corresponds to the plus or minus timing error 242 or 244 suchthat the sample rate conversion module 138 adjusts the sample rateconversion based on the plus or minus timing error, therebysubstantially illuminating the timing error 242 and/or 244 prior todecoding.

FIG. 17 is a schematic block diagram of a sample rate conversion module138 that includes a sampling module 250, a low pass filter 252, a linearsample rate conversion module 254, and a sigma-delta modulator 255. Thesampling module 250 samples a digital input signal 256, which has afirst sampling rate, to produce a digitally sampled single 258. At aminimum, the sampling module 250 over samples the digital input signalin accordance with the Nyquist rate. In one embodiment, the digitalinput signal may include an in-phase signal component of a basebandradio signal and a quadrature signal component of the baseband radiosignal. Accordingly, the digital up-sampled signal 258 would include anup-sampled I component and an up-sampled Q component.

The low pass filter 252 filters the digital sampled signal 258 toproduce a digitally filtered signal 260. Note that in one embodiment,the sampling module 250 and low pass filter 252 may be implemented via acascaded integrated cone filter 264.

The linear sample rate conversion module 254 converts the digitallyfiltered signal 260 into a sample rate adjusted digital signal 262 basedon a control feedback signal 264. In one embodiment, the sigma-deltamodulator 255 may generate the control feedback signal 264 based on aratio between the rate of the sample rate adjusted digital signal 262and the rate of the digital input signal 256. As one of ordinary skillin the art will appreciate, the rate of the sample rate adjusted digitalsignal 262 may be greater than or less than the rate of the digitalinput signal 256. With such a sample rate converter, few bits are neededby using a time averaging of the sample values as opposed to usingspecific sample values.

In another embodiment, the linear sample rate conversion module 254functions to pass, as a sample of the sample rate adjusted digitalsignal, a sample of the digitally filtered signal when, for the sampleof the sample rate adjusted digital signal, the control feedback signalhas a value that is within a first value range, e.g., plus or minus agiven percentage of the sample rate. The linear sample rate conversionmodule 254 also functions to determine, as the sample of the sample rateadjusted digital signal, a sample value based on the current sample ofthe digitally filtered signal and a previous sample of the digitallyfiltered signal when, for the sample of the sample rate adjusted digitalsignal, the control feedback signal has a value that is outside thefirst value range. The first value range corresponds to the amount ofdifference between the digitally filtered signal in time with respect toa desired sample point of the sample rate adjusted digitally signal. Forinstance, the first value range may correspond to a difference of plusor minus 10%, or less.

The linear sample rate conversion module 254 may determine the samplevalue by multiplying the previous sample value with the value of thecontrol feedback signal to produce a first product. The linear samplerate conversion module then subtracts the value of the control feedbacksignal from a maximum value of the feedback error signal to produce acomplimentary error feedback signal. The linear sample rate conversionmodule then multiples the current sample with the complimentary errorfeedback signal to produce a second product. The linear sample rateconversion module then sums the first and second products to produce asum and divides the sum by the maximum value of the feedback errorsignal to produce the sample value. Generally, the linear sample rateconversion module 252 is performing a linear function to determine thesample value, where the linear function may correspond to Y=mX+b.

As one of ordinary skill in the art will appreciate, a linearinterpolator may be implemented using the linear sample rate conversionmodule 254 and the sigma-delta modulator 255. The linear sample rateconversion module is operably coupled to sample a digital signal inaccordance with a control feedback signal. The sigma-delta modulator isoperably coupled to produce the control feedback signal based on aninterpolation ratio. In one embodiment, the interpolation ratio is aratio between the input sample rate and the output sample rate of thelinear interpolator.

FIG. 18 illustrates a schematic block diagram of another embodiment of asample rate converter 170, which may be used for the sample rateconverter 138 of FIG. 8. The sample rate converter 270 includes asampling module 272, a determining module 274, and an output module 276.The sampling module 272 is operably coupled to up-sample an input signal278 based on a sampling rate 280 to produce a sample 284 of a digitalsampled signal 286. Note that in one embodiment the input signal 278 maycorrespond to the digital baseband signal 152 of FIG. 8. In anotherembodiment, the input signal 278 may be an analog signal or digitalsignal. Note that if the input signal 278 is a digital signal, thesampling module 272 further includes a digital low pass filter to filterthe digital signal thereby producing the corresponding input signal 278.The sampling rate 280 may be any integer value to produce the digitalsampled signal 286. For example, the sampling rate 280 may be any 2^(N)up-sampling rate or an integer multiple sampling rate of the rate of theinput signal 278.

The determining module 274 is operably coupled to determine an errorterm 288 from the sample 284 of the digital sampled 286. The determiningmodule 274 determines, for a given sample of the digital sampled signal286, whether a sample of the digital sample signal has an error termwithin a first value range. The determining module 274 determines theerror term based on known properties of the digital sampled signal 286in comparison with the particular sample 284. If the particular sample284 does not coincide with the known properties of the digital sampledsignal 286, the error term 288 is generated.

The output module 276 is operably coupled to pass the sample 284 as anoutput sample 290 of the sample rate converted digital signal 292 whenthe error term 288 is within a first value range. The output module 276is also operably coupled to determine a value for the output sample 290of the sample rate conversion digital signal 292 from the sample 284 ofthe digital sampled signal 286 based on the error term 288. In oneembodiment, the output module 276 determines the sample 290 from thesample 284 an error term 288 by multiplying the previous sample with theerror term to produce a first product. The output module then subtractsthe error term from a maximum value of the error term to produce acomplimentary error term. The output module 276 then multiples thesample with the complimentary error term to produce a second product.The output module 276 then sums the first and second products to producea sum and then divides the sum by the maximum value of the error term toproduce the sample value.

FIGS. 19A-19D illustrate the functionality of the sample rate convertersof FIGS. 8, 17 and 18. As shown in FIG. 19A, a sample rate converterinput signal 256 has a first sampling rate of eight. As such, asinusoidal signal will have 8 sampling points per period. As is alsoshown in FIG. 19A, the input signal 256 is up-sampled at a particularrate (e.g., 16) to produce a plurality of samples 300 of the digitallyup-sampled signal. As one of ordinary skill in the art will appreciate,the sample rate of 8 and oversampling of 16 are mere examples ofnumerous values that could be used.

FIG. 19B illustrates the ideal sample rate converted output signal 304where the new sample rate sampling points 302 are shown. In thisexample, the ideal sample rate converted output signal 304 has 6sampling points per period. As one of ordinary skill in the art willappreciate, the sample rate of 6 is a mere example of the numerousvalues that could be used.

As is also shown in FIG. 19B, the old sampling rate points 308 are shownby X and the new sampling rate points 302 are shown as zeros. Thesamples 306 of the digitally filtered signal are shown by lines up tothe magnitude of the digitally filtered signal 260. As is further shown,the details of two sample rate conversion points will be furtherillustrated in FIGS. 19C and 19D.

In FIG. 19C, the old sampling rate points 308 are shown to provide aborder around the new sampling point 302. As is also shown, one of thesamples 306 of the digital filtered signal occurs, in time, with the newsample rate sampling points 302. In this instance, the error term wouldbe zero since the difference between the particular sample 306 and thenew sample rate sampling point 302 are aligned in time. As such, thesample outputted for the ideal sample rate converted signal 304 is thesample 306.

FIG. 19D illustrates the new sampling point occurring in time, betweentwo up-sampled points 306. In this instance, an error term is determinedand, based on a linear function and the adjacent samples 306, the sampleoutputted for the ideal sample rate converted signal 304 is determined.

As one of ordinary skill in the art will appreciate, the term“substantially” or “approximately”, as may be used herein, provides anindustry-accepted tolerance to its corresponding term and/or relativitybetween items. Such an industry-accepted tolerance ranges from less thanone percent to twenty percent and corresponds to, but is not limited to,component values, integrated circuit process variations, temperaturevariations, rise and fall times, and/or thermal noise. Such relativitybetween items ranges from a difference of a few percent to magnitudedifferences. As one of ordinary skill in the art will furtherappreciate, the term “operably coupled”, as may be used herein, includesdirect coupling and indirect coupling via another component, element,circuit, or module where, for indirect coupling, the interveningcomponent, element, circuit, or module does not modify the informationof a signal but may adjust its current level, voltage level, and/orpower level. As one of ordinary skill in the art will also appreciate,inferred coupling (i.e., where one element is coupled to another elementby inference) includes direct and indirect coupling between two elementsin the same manner as “operably coupled”. As one of ordinary skill inthe art will further appreciate, the term “compares favorably”, as maybe used herein, indicates that a comparison between two or moreelements, items, signals, etc., provides a desired relationship. Forexample, when the desired relationship is that signal 1 has a greatermagnitude than signal 2, a favorable comparison may be achieved when themagnitude of signal 1 is greater than that of signal 2 or when themagnitude of signal 2 is less than that of signal 1.

The preceding discussion has presented a handheld device thatincorporates a radio signal decoder integrated circuit optimizedinterface with a digital audio processing integrated circuit. As one ofaverage skill in the art will appreciate, other embodiments may bederived from the teaching of the present invention without deviatingfrom the scope of the claims.

1. A sample rate converter comprises: an upsampling module operablycoupled up-sample a digital input signal having a first rate to producea digitally up-sampled signal; a low pass filter operably coupled to lowpass filter the digitally up-sampled signal to produce a digitallyfiltered signal at an up-sampled rate; and linear sample rate conversionmodule operably coupled to convert the digitally up-sampled signal intoa sample rate adjusted digital signal having a second rate based on ancontrol feedback signal and a linear function, wherein a relationshipbetween the first rate and the second rate is a non-power of two.
 2. Thesample rate converter of claim 1, wherein the low pass filter and theupsampling module comprises: a cascaded integrated comb (CIC) filteroperably coupled to filter and upsample the digital input signal toproduce the digitally upsampled digital signal at the second data rate.3. The sample rate converter of claim 1 further comprises: the digitalinput signal including an in-phase signal component of a baseband radiosignal; and the rate adjusted digital signal including a rate adjustedin-phase signal component and a rate adjusted derivative in-phase signalcomponent.
 4. The sample rate converter of claim 1 further comprises:the digital input signal including a quadrature signal component of abaseband radio signal; and the rate adjusted digital signal including arate adjusted quadrature signal component and a rate adjusted derivativequadrature signal component.
 5. The sample rate converter of claim 1,wherein the linear sample rate conversion module further functions to:pass, as a sample of the sample rate adjusted digital signal, a sampleof the digitally up-sampled signal when, for the sample of the samplerate adjusted digital signal, the control feedback signal has a valuethat is within a first value range; and determine, as the sample of thesample rate adjusted digital signal, a sample value based on a currentsample of the digitally up-sampled signal and a previous sample of thedigitally up-sampled signal when, for the sample of the sample rateadjusted digital signal, the control feedback signal has a value that isoutside the first value range.
 6. The sample rate converter of claim 5,wherein the determining the sample value comprises: multiplying theprevious sample with the value of the control feedback signal to producea first product; subtracting the value of the control feedback signalfrom a maximum value of the feedback error signal to produce acomplimentary error feedback value; multiplying the current sample withthe complimentary error feedback value to produce a second product;summing the first and second products to produce a sum; and dividing thesum by the maximum value of the feedback error signal to produce thesample value.
 7. A sample rate converter comprises: a sampling moduleoperably coupled to sample, at a sampling rate, an input signal toproduce a digital sampled signal at the sampling rate; determiningmodule operably coupled to determine whether, for a given sample of asample rate conversion digital signal at a second rate, a sample of thedigital sampled signal has an error term within a first value range,wherein a relationship between the sampling rate and the second rate isa non-power of two; and outputting module operably coupled to: pass thesample of the digital sampled signal as the given sample when the errorterm is within the first value range; and determine a value for thegiven sample based on the sample of the digital sampled signal, aprevious sample of the digital sampled signal, and the error term whenthe error term is not within the first value range.
 8. The sample rateconverter of claim 7, wherein the input signal comprises an analogsignal.
 9. The sample rate converter of claim 7 further comprises: theinput signal including an digital signal at a first rate, wherein arelationship between the first rate and the sampling rate is an integer;and the sampling module including a digital low pass filter to low passfilter the digital signal to produce the input signal.
 10. The samplerate converter of claim 7, wherein the output module further functionsto: multiply the previous sample with the error term to produce a firstproduct; subtract the error term from a maximum value of the error termto produce a complimentary error term; multiply the sample with thecomplimentary error term to produce a second product; sum the first andsecond products to produce a sum; and divide the sum by the maximumvalue of the error term to produce the sample value.
 11. A digitalFrequency Modulation (FM) decoder comprises: a low noise amplifieroperably coupled to amplify a received FM signal to produce an amplifiedFM signal; mixing module operably coupled to convert the amplified FMsignal into a low intermediate frequency (IF) signal based on a localoscillation; analog to digital conversion module operably coupled toconvert the low IF signal into a digital low IF signal; digital basebandconversion module operably coupled to convert the digital low IF signalinto a digital baseband signal; a sample rate converter operably coupledto adjust the digital baseband signal from a first rate to a second ratebased on a feedback error signal to produce a digital FM encoded signal;demodulation module operably coupled to demodulate the digital FMencoded signal to produce a digital FM composite signal; channelseparation module operably coupled to separate a left channel signal anda right channel signal from the digital FM composite signal; and anerror sensing module operably coupled to produce the feedback errorsignal based on the digital FM composite signal.
 12. The digital FMdecoder of claim 11, wherein the sample rate converter comprises: anupsampling module operably coupled to up-sample a digital basebandsignal having a first rate to produce a digitally up-sampled signal; alow pass filter operably coupled to low pass filter the digitallyup-sampled signal to produce a digitally filtered signal at anup-sampled rate; and linear sample rate conversion module operablycoupled to convert the digitally up-sampled signal into the digital FMencoded signal having the second rate based on an control feedbacksignal and a linear function, wherein a relationship between the firstrate and the second rate is a non-power of two.
 13. The digital FMdecoder of claim 12, wherein the low pass filter and the upsamplingmodule comprises: a cascaded integrated comb (CIC) filter operablycoupled to filter and upsample the digital baseband signal to producethe digitally upsampled digital signal at the second data rate.
 14. Thedigital FM decoder of claim 12, wherein the sample rate converterfurther comprises: the digital baseband signal including an in-phasesignal component and a quadrature component; and the digital FM encodedsignal including a rate adjusted in-phase signal component, a rateadjusted derivative in-phase signal component, a rate adjustedquadrature signal component, and a rate adjusted derivative quadraturesignal component.
 15. The digital FM decoder of claim 12, wherein thelinear sample rate conversion module further functions to: pass, as asample of the digital FM encoded signal, a sample of the digitallyup-sampled signal when, for the sample of the sample rate adjusteddigital signal, the control feedback signal has a value that is within afirst value range; and determine, as the sample of the digital FMencoded signal, a sample value based on a current sample of thedigitally up-sampled signal and a previous sample of the digitallyup-sampled signal when, for the sample of the sample rate adjusteddigital signal, the control feedback signal has a value that is outsidethe first value range.
 16. The digital FM decoder of claim 15, whereinthe determining the sample value comprises: multiplying the previoussample with the value of the control feedback signal to produce a firstproduct; subtracting the value of the control feedback signal from amaximum value of the feedback error signal to produce a complimentaryerror feedback value; multiplying the current sample with thecomplimentary error feedback value to produce a second product; summingthe first and second products to produce a sum; and dividing the sum bythe maximum value of the feedback error signal to produce the samplevalue.
 17. The digital FM decoder of claim 11, wherein the sample rateconverter comprises: a sampling module operably coupled to sample, at asampling rate, the digital baseband signal to produce a digital sampledsignal at the sampling rate; determining module operably coupled todetermine whether, for a given sample of the digital FM encoded signal,a sample of the digital sampled signal has an error term of the feedbackerror signal within a first value range, wherein a relationship betweenthe sampling rate and the second rate is a non-power of two; andoutputting module operably coupled to: pass the sample of the digitalsampled signal as the given sample when the error term is within thefirst value range; and determine a value for the given sample based onthe sample of the digital sampled signal, a previous sample of thedigital sampled signal, and the error term.
 18. The digital FM decoderof claim 17, wherein the sampling module comprises: a digital low passfilter to low pass filter the digital baseband signal prior toup-sampling the digital baseband signal.
 19. The digital FM decoder ofclaim 17, wherein the output module further functions to: multiply theprevious sample with the error term to produce a first product; subtractthe error term from a maximum value of the error term to produce acomplimentary error term; multiply the sample with the complimentaryerror term to produce a second product; sum the first and secondproducts to produce a sum; and divide the sum by the maximum value ofthe error term to produce the sample value.
 20. A sample rate convertercomprises: an sampling module operably coupled to sample a digital inputsignal having a first rate to produce a sampled digital signal having asecond rate; a low pass filter operably coupled to filter the sampleddigital signal to produce a filtered digital signal; a linear samplerate conversion module operably coupled to sample the filtered digitalsignal in accordance with a control feedback signal to produce a samplerate adjusted digital signal having a third rate; and a sigma-deltamodulator operably coupled to produce the control feedback signal basedon the first and third rates.
 21. The sample rate converter of claim 20further comprises: a cascaded integrated comb filter functioning as thesampling module and the low pass filter.
 22. The sample rate converterof claim 20, wherein the linear sample rate conversion module furtherfunctions to: pass, as a sample of the sample rate adjusted digitalsignal, a sample of the digitally up-sampled signal when, for the sampleof the sample rate adjusted digital signal, the control feedback signalhas a value that is within a first value range; and determine, as thesample of the sample rate adjusted digital signal, a sample value basedon a current sample of the digitally up-sampled signal and a previoussample of the digitally up-sampled signal when, for the sample of thesample rate adjusted digital signal, the control feedback signal has avalue that is outside the first value range.
 23. The sample rateconverter of claim 22, wherein the determining the sample valuecomprises: multiplying the previous sample with the value of the controlfeedback signal to produce a first product; subtracting the value of thecontrol feedback signal from a maximum value of the feedback errorsignal to produce a complimentary error feedback value; multiplying thecurrent sample with the complimentary error feedback value to produce asecond product; summing the first and second products to produce a sum;and dividing the sum by the maximum value of the feedback error signalto produce the sample value.
 24. A linear interpolator comprises: alinear sample rate conversion module operably coupled to sample adigital signal in accordance with a control feedback signal; and asigma-delta modulator operably coupled to produce the control feedbacksignal based on an interpolation ratio.
 25. The linear interpolator ofclaim 24, wherein the linear sample rate conversion module furtherfunctions to: pass, as a sample of a sample rate adjusted digitalsignal, a sample of a digitally up-sampled signal when, for the sampleof the sample rate adjusted digital signal, the control feedback signalhas a value that is within a first value range; and determine, as thesample of the sample rate adjusted digital signal, a sample value basedon a current sample of the digitally up-sampled signal and a previoussample of the digitally up-sampled signal when, for the sample of thesample rate adjusted digital signal, the control feedback signal has avalue that is outside the first value range.
 26. The linear interpolatorof claim 25, wherein the determining the sample value comprises:multiplying the previous sample with the value of the control feedbacksignal to produce a first product; subtracting the value of the controlfeedback signal from a maximum value of the feedback error signal toproduce a complimentary error feedback value; multiplying the currentsample with the complimentary error feedback value to produce a secondproduct; summing the first and second products to produce a sum; anddividing the sum by the maximum value of the feedback error signal toproduce the sample value.